This invention relates to a small hydraulic pump with a plunger driven by an eccentric cam, and more particularly to an improvement of the small hydraulic pump.
There are provided various plungers, cylinders, supply valves and discharge valves for a small hydraulic pump of this type. Recently, a small hydraulic pump, which is effective in miniaturization of a hydraulic pump, and in improvement of the pumping efficiency thereof, has been disclosed by Japanese Utility Patent Application (OPI) No. 79477/2988 (the term "OPI" as used herein means an "unexamined published application").
One example of the small hydraulic pump will be described with reference to FIG. 1 (Prior Art).
In the hydraulic pump, a plunger 4 is driven by a bearing 3 fitted on an eccentric cam 2 on a drive shaft 1. A suction valve comprises a ball valve 5 positioned in the upper end portion of a cylinder, and a spring 6 urging the ball valve 5 so as to close the suction valve. An ejection valve is made up of a ball valve 8 set in an ejection port, and a spring 9 urging the ball valve 8 so as to close the ejection valve.
When the plunger is moved downwardly, the ball valve 5 is moved downwardly against the spring 6 by the negative pressure in the cylinder, opening the suction valve. As a result, operating oil is sucked into the cylinder through a suction port 10. When the plunger is moved upwardly, the ejection valve is opened by the pressure in the cylinder, so that the high pressure oil is discharged from the cylinder through the ejection port.
In the hydraulic pump, the negative pressure in the cylinder moves the ball valve 5 against the valve closing force of the spring 6 to suck the operating oil into the cylinder, and therefore, the inflow resistance is high and the negative pressure in the cylinder is also high. Hence, suction of the operating oil is not smooth and gas is released from the operating oil. As the plunger is moved upwardly, the pressure in the cylinder is increased, so that the gas thus released is dissolved into the operating oil, with the result that the pumping efficiency is decreased accordingly.
Another example of the conventional small hydraulic pump will be described with reference to FIG. 2 (Prior Art).
In the hydraulic pump, a plunger 33 is driven by a bearing fitted on an eccentric cam 32 which is fixedly mounted on a pump shaft 31. An ejection valve comprises a ball valve 34 positioned in the upper end portion of a cylinder, and a spring 35 urging the ball valve 34 to close the ejection valve. A suction valve comprises a ball valve 37 set in a suction port 36, and a spring 38 urging the ball valve 37 to close the suction valve.
When the plunger 33 is moved downwardly, the ball valve 37 is pulled upwardly by the negative pressure in the cylinder against the elastic force of the spring 38, opening the suction valve. As a result, operating oil is sucked into the pressurizing chamber P of the cylinder through the suction port 36. When the plunger 33 is moved upwardly, the ejection valve is opened by the pressure in the cylinder, so that the high pressure oil is discharged from the pressurizing chamber P through the ejection port d.
In the hydraulic pump described above, the drive shaft is vibrated by the high speed rotation of the eccentric cam 32, and the vibration is transmitted to the cylinder block B. Furthermore, the pressure in the cylinder changes at high frequency so that the cylinder block B vibrates. In addition, the ball valves are moved to and from the valve seats at high frequency, also causing the cylinder block B to vibrate. Since the cylinder block B is fixed directly to the pump housing H, the vibration of the cylinder block B is transmitted directly to the pump housing H, thus vibrating equipment related to the hydraulic pump, or producing vibrational sounds.